What is sympathoexcitation?

Sympathoexcitation: Definition and Clinical Significance

Sympathoexcitation refers to increased activity of the sympathetic nervous system, characterized by heightened sympathetic outflow to target organs, which can influence cardiovascular function and contribute to various pathological conditions.

Physiological Basis

Sympathoexcitation involves increased sympathetic nerve activity that affects the cardiovascular system in several ways:

• Mechanism of Action: Acts as a "low-pass filter" on heart rate variability (HRV), allowing primarily low-frequency fluctuations to influence heart rate while diminishing the impact of higher-frequency perturbations 1

• Frequency Domain Characteristics:

  • Primarily influences a narrow band around 0.1 Hz (approximately 6 cycles/min) in heart rate variability measurements 1
  • During sympathetic predominance, low-frequency components become relatively more prominent compared to high-frequency components 1

• Paradoxical Effect on HRV:

  • Despite relative predominance of low frequencies during sympathetic activation, the absolute power of all frequency components actually decreases 1
  • This occurs due to global reduction in HRV caused by parasympathetic withdrawal and increased heart rate 1

Measurement and Assessment

Sympathoexcitation can be assessed through several methods:

• Heart Rate Variability Analysis:

  • The relative proportion (not absolute power) of low-frequency to high-frequency components provides an approximate indication of sympathetic modulation 1
  • Must control for respiratory artifacts (slow breaths) that can confound interpretation 1

• Gold Standard Measurement:

  • Direct measurement via muscle sympathetic nerve activity (MSNA) using intraneural microelectrodes 2
  • Can be measured through single unit recordings of postganglionic sympathetic firing 2

• Other Assessment Methods:

  • Plasma catecholamine measurements (norepinephrine, epinephrine) 1
  • Low-frequency power of blood pressure variability 1

Clinical Implications

Sympathoexcitation plays important roles in various clinical conditions:

• Perioperative Setting:

  • Associated with post-induction hypotension in high-risk patients 1
  • May contribute to organ injury through direct cellular mechanisms and inflammatory pathways 1
  • Can trigger DNA damage and suppress p53 levels through β-adrenergic pathways 1

• Chronic Conditions:

  • Serves as a hallmark feature in aging and obesity 3
  • Contributes to development of cardiovascular diseases including hypertension and heart failure 3
  • Present in COPD patients, potentially contributing to increased cardiovascular risk 2

• Heart Failure:

  • Excessive sympathoexcitation characterizes chronic heart failure 4, 5
  • In heart failure patients, sympathoexcitation is related to decreased tidal volume and attenuated sympathoinhibitory effect of lung inflation reflex 5

Clinical Pitfalls and Considerations

When evaluating sympathetic activity, important caveats include:

• Measurement Interpretation:

  • Absolute power in low-frequency region should NOT be used as an index of sympathetic activity 1
  • With extreme sympathetic activation, overall variability becomes so small that low-frequency component may no longer be measurable 1

• Confounding Factors:

  • Respiratory patterns significantly influence sympathetic measurements 1, 5
  • Slow breathing can create spurious low-frequency components that confound interpretation 1
  • Standardization of experimental conditions is crucial for accurate assessment 1

• Therapeutic Considerations:

  • During exercise, reduction of arterial baroreceptor reflex sensitivity appears to result primarily from vagal withdrawal rather than increased sympathetic activity 6
  • Beta-blocker therapy may be considered for managing increased sympathetic nerve activity in certain conditions 2

Understanding sympathoexcitation is essential for comprehending autonomic regulation in health and disease, particularly in cardiovascular conditions where autonomic imbalance contributes to pathophysiology and clinical outcomes.